The invention is directed to a method for determining various parameters of a lightguide preform. In particular, the invention is directed to a non-contact, non-destructive technique for measuring the refractive index profile of such a preform. A lightguide preform is comprised of a substantially cylindrical glass core surrounded by a glass cladding which may be fabricated by several processes well known in the art. These processes involve the thermochemical production of glass from appropriate glass precursor vapors.
The refractive index profile of an optical fiber is substantially the same as the profile of the preform from which it was drawn. Therefore, deviations from the refractive index profile n(r) of the preform may result in unacceptable transmission characteristics of the fiber drawn therefrom. Thus, it becomes most advantageous to determine the refractive index profile of the preform prior to drawing fiber therefrom to avoid the expense of producing kilometers of fiber that would be unacceptable for its intended use.
One non-destructive technique for measuring the refractive index of a lightguide preform is described in U.S. Pat. No. 4,227,806 to Watkins which issued on Oct. 14, 1980 and is incorporated by reference herein. In that patent a laser beam scans an optical fiber preform and the deflection angle .theta. thereof is measured as the beam exits the preform. The deflection angle .theta. is plotted versus the incident beam position and that plot is integrated to provide a curve that is compared to theoretically developed plots for refractive index profiles characterized by known parameters.
The technique developed by Watkins has since been improved in several important respects. Rather than determining certain index profile parameters by comparing the integrated .theta.-curve to equivalent curves computed for refractive index profiles with known parameters, the index profiles obtained with the improved technique are directly computed from the measured deflection angles .theta.. If .theta.(x) is the angle of refraction observed for a beam incident on the preform a distance x from the preform axis, then the radius of closest approach to the preform axis r(x) of that beam is given by ##EQU1## where R is the preform radius and t is the variable of integration. The refractive index n[r(x)] at radius r(x) is given as ##EQU2## where n(R) is the refractive index of the preform at its surface. Using these equations to establish the refractive index profile of an optical fiber preform is preferable to using Watkin's earlier approach since these equations are applicable to a much larger class of index profiles.
While the technique as outlined up to this point has been found to be effective and useful for determining the refractive index profiles of lightguide preforms it has one deficiency. The measurement process requires that an operator manually focus the system prior to scanning the laser beam past the preform. This manual focusing process amounts to an undesirable machine-operator interaction because (a) the quality of an index profile measurement depends at least partially on the skill of the operator affecting the focusing, and (b) without the focusing process the measurement could be totally automated. Clearly, eliminating this manual focusing procedure would be desirable.